If the command returns a value near the allowed local port range (source port for client connections), then you might be running into port exhaustion. To reduce port exhaustion, try one of these solutions:

Increase the operating system local (ephemeral) port range by running this command:

net.ipv4.ip_local_port_range = 1025 61000

Add ephemeral ports for new connections by allocating more elastic IPs to the NAT instance, or by increasing the number of NAT instances for internet-bound traffic.

Resolve any application-level issues that drain the available connections.

Network ACL rules

Confirm that the network ACL allows inbound traffic from the ephemeral port range (1024-65535). If the network ACL allows only a subset of the ephemeral port range, and the instances in the private subnet use a source port outside of that range, then traffic is dropped.

Note: If you're using a NAT gateway instead of a NAT instance, use the CloudWatch ErrorPortAllocation metric to verify if source ports are exhausted. For more information on this metric, see Amazon VPC NAT Gateway Metrics and Dimensions.